Due to increased power generation by unsteady renewable sources like wind or solar existing gas turbine based power plants are increasingly used to balance power demand and to stabilize the grid. Thus improved operational flexibility is required. This implies that gas turbines are often operated at lower load than the base load design point, i.e. at lower combustor inlet and firing temperatures.
At the same time, emission limit values and overall emission permits are becoming more stringent, so that it is required to operate at lower emission values, keep low emissions also at part load operation and during transients, as these also count for cumulative emission limits.
State-of-the-art combustion systems are designed to cope with a certain variability in operating conditions, e.g. by adjusting the compressor inlet mass flow or controlling the fuel split among different burners, fuel stages or combustors. However, this is not sufficient to meet the new requirements.
To further reduce emissions and to increase operational flexibility sequential combustion has been suggested. Depending on the operating conditions, in particular on the hot gas temperature of a first combustion chamber it can be advantageous to cool the hot gases before they are admitted to a second burner 5 (also called sequential burner). Such cooling has been described in DE 10312971 A1. It can be advantageous to allow fuel injection and premixing of the injected fuel with the hot flue gases of the first combustor in the second burner.
Operation methods for steady state at base load have been described for sequential combustion. However, when switching on or off the second stage of a sequential combustion arrangement flame instabilities and increased emissions can occur due to a shift of fuel flow form the first to the second stage or vice versa. Due to this shift of fuel flow the local fuel to combustion air or fuel to oxidizer ratio can shift out of the design range for clean stable combustion.